This invention relates to turbomachinery in general and to a flow conditioner for a low pressure inlet in a steam turbine in particular.
A low pressure (LP) inlet in a steam turbine casing is designed to transfer working fluid (steam) from the power plant piping to the LP turbine unit. The steam may be generated in, for example, a heat recovery steam generator in a combined cycle power plant. It has been found that obstructions and discontinuities in the inlet passage surfaces of the steam turbine casing tend to increase the energy loss of the steam. This lost energy is not available for use by the steam turbine, thereby reducing the overall efficiency of the turbine.
One such surface discontinuity is a back-facing step (or back step) created by a pipe flange of circular cross-section joined to a polygonal, faceted inlet structure of the casing. Other discontinuities arise in that the faceted inlet structure transitions from a polygonal cross section adjacent the pipe flange to a substantially rectangular cross section downstream of the flange (in the flow direction) via a plurality of flat surfaces.
Prior methods of reducing energy loss in the LP inlet are summarized below:
(1) Metering flow upstream of the inletxe2x80x94while this may reduce losses in the inlet proper, metering of the flow anywhere in the flow path generates energy loss. For other reasons, it may be necessary to meter the flow upstream of the inlet step and thereby mitigate losses at the step. In this case, there will be no additional loss. However, metering of the flow is not an effective method if used to specifically target energy loss associated with a back-facing step.
(2) Contouring the inlet wallsxe2x80x94this technique redistributes the velocity profile at the 1st stage nozzle, but there is no method of reacquiring the lost energy associated with the back step to the LP inlet.
(3) Using larger pipe diameterxe2x80x94this is an effective method of slowing the steam flow velocity since losses are proportional to the square of velocity. The disadvantage of this method, however, is that the cost of the piping increases with diameter. Furthermore, this method may influence the velocity profile at the 1st stage nozzle.
There remains a need for a technique or device that conceals the back step from the flow path and that has either beneficial or no influence on the 1st stage velocity profile.
In an exemplary embodiment of this invention, a xe2x80x9cflow conditionerxe2x80x9d constructed of an appropriate material is provided that may be inserted (as a retrofit), or designed into the LP inlet of a steam turbine casing to reduce energy loss of the working fluid. The flow conditioner in the exemplary embodiment conceals the back facing step generated at the interface of the round inlet pipe flange and the polygonal LP inlet, and otherwise presents a smoothly contoured surface to the flowing fluid in the inlet transition area. While the exact profile of the flow conditioner necessarily varies for each application, the device as described herein includes a round axially oriented inlet edge, i.e., a cylindrical inlet portion concentric with the longitudinal axis of the LP inlet. Downstream of the cylindrical inlet, the flow conditioner presents a pair of smoothly tapered (downwardly and outwardly in the flow direction) surfaces, or xe2x80x9cwings,xe2x80x9d diametrically opposed to each other, and each initially spanning about 180xc2x0 of the cylindrical inlet. These surfaces each have side edges that taper towards each other, so that the terminal edge of each wing, when viewed in plan, circumscribe an angle of only about 25xc2x0 relative to the center axis. The generally parallel terminating edges of each wing correspond to the width dimension of the two shorter sides of the rectangular cross section further within the LP inlet. It will be appreciated that the side edges of one wing merge smoothly into the side edges of the other wing, and lie in angled planes such that they seat on inwardly tapered flat surfaces of the LP inlet. The smoothly contoured wings thus not only conceal the back facing step at the round inlet pipe flange, but also the corners or edges where the flat surfaces of the LP inlet interface or join to each other, thus smoothing the flow through the inlet.
In its broader aspects, the invention thus relates to a flow conditioner for a steam turbine inlet comprising an annular ring adapted to seat in a flange portion of an inlet in a turbine casing and a pair of wing portions extending from the annular ring in a flow direction, away from the annular ring.
In another aspect, the invention relates to a turbine inlet casing having a main body portion and a steam inlet in communication with the main body portion, the steam inlet including an annular flange and a transition portion extending between the annular flange and the main body portion, the annular flange and transition portion generating a back facing step at an interface thereof; and a flow conditioner inserted within the inlet concealing the back facing step to thereby smooth flow through the inlet.
In still another aspect, the invention relates to a method of smoothing flow through an inlet of a steam turbine inlet casing wherein the inlet includes a circular flange and a non-circular transition portion extending between the annular flange and a main body portion of the steam turbine casing, the method comprising a) identifying surface discontinuities that disrupt flow of working fluid through the steam inlet; b) providing a flow conditioner that is shaped to conceal one or more of the surface discontinuities; and c) inserting the flow conditioner into the inlet.
The invention will now be described in detail in connection with the drawings.